Direct modulated modified vertical-cavity surface-emitting lasers and method

ABSTRACT

A laser system having separately electrically operable cavities for emitting modulated narrow linewidth light with first, second and third mirror structures separated by a first active region between the first and the second and by a second active region between the second and the third. The second mirror structure has twenty of more periods of mirror pairs.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. patentapplication Ser. No. 12/710,156, filed Feb. 22, 2010 and titled “DIRECTMODULATED MODIFIED VERTICAL CAVITY SURFACE EMITTING LASERS,” whichclaims the benefit under 35 U.S.C. §119(e) of Provisional ApplicationNo. 61/208,200 filed on Feb. 20, 2009 for “DIRECT MODULATED MODIFIEDVERTICAL CAVITY SURFACE EMITTING LASERS,” each of which are incorporatedherein by reference in their entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with Government support under contractW31P4Q-07-C-0284 awarded by the U.S. Army. The Government has certainrights in the invention.

BACKGROUND

The present invention relates to a modulated intensity output solidstate laser and, more particularly, to a modulated intensity outputvertical cavity surface emitting solid state laser.

Large numbers of closely spaced lateral circuit interconnections,extending between various portions of individual integrated circuitchips, between various integrated circuit chips mounted on a printedcircuit board, and between various printed circuit boards mounted in asystem, that can each transmit large numbers of signal symbols withextreme rapidity are increasingly needed. These interconnections areneeded to move, between selected locations, the large amounts of datagenerated by very fast signal processors that appear on signal bussesfor transmitting signal symbols representing such data, data that is tobe received and sent by those processors and by various related datareceiving, using, generating and transmitting devices.

As chip area and board mounted component density increases, the numbersof unavoidable, but unwanted, electrical circuit couplings, orparasitics, will most certainly increase substantially. Dynamic powerdissipation in on-chip and off-chip circuits for operating circuitinterconnections comprises the vast majority of total power consumed.Dynamic dissipation scales linearly with switching speed, and so powerconsumption per line in electrical interconnections can be expected tosoon outstrip that of their optical interconnection counterparts wherethe power dissipation is essentially independent of signal path lengthover those interconnections. Hence, there will be transitions in thefuture to optical interconnection based system architectures.

These optical interconnection arrangements will require low cost, lowpower, directly modulated, high-reliability, single-chip laser sourcesand source arrays operating at data rates in excess of 17 Gbps, now, butcapable of reaching 100 Gbps in the future, to meet the demands ofexisting, and emerging future, serial chip and board data communicationsrequirements. Such required capabilities for the laser sources lead todifficult requirements to be met by those sources in terms of powerdissipation, reliability, and interconnection spatial densities.

Single lasers and one dimensional and two-dimensional laser arrays areneeded for fiber optic links, board-to-board and chi-to-chip links. Eachlaser should dissipate less than 2 to 5 mW/laser. Reliability must begreater than 100,000 hours (10 years) at a minimum. Device-to-deviceuniformity needs to be high (variations being less than 5%), and deviceaging characteristics must be sufficiently slow to eliminate any needfor power monitoring. Low device lasing thresholds and high modulationefficiencies will be required to minimize electrical power drains in thelaser driver arrays. In addition, in the case of intra-chip opticalinterconnects, thermal dissipations pose a particularly challengingproblem as the components may be expected to operate at ambienttemperatures in excess of 80 C. This not only will have a significantimpact on device intrinsic bandwidth, but on device reliability as well.

Vertical cavity surface emitting lasers (VCSELs) have been found to besuitable laser sources for short transmission distance optical networkswith 10 Gbps VCSELs being the laser devices with the largest modulationrates commercially available today. VCSELs thus are the dominant lightemission source for short transmission distance optical interconnectionarrangements and local area networks because of their large modulationrate capabilities, low power consumption, spatially dense device arrayintegration, and low cost manufacturing of those devices when made insufficiently large numbers.

VCSEL sources that are directly modulated to correspondingly vary theemitted light intensity at large modulation rates offer a substantialdecrease in cost over the typical alternative, a CW laser operated inconjunction with an external adjacent electro-absorption modulator. Animportant figure of merit for modulation rates in lasers is the −3 dBsmall-signal modulation bandwidth that is defined as the point at whichthe modulated optical output, measured as a function of frequency, isreduced to half of its low modulation rate value. A variety of methodshave been used to achieve greater modulation rates of light intensitiesemitted by VCSELs. These have included use of metal contacts on polymerlayers as well as ion implantation to reduce device capacitance toachieve small-signal modulation bandwidths of 16 to 20 GHz. Althoughstate-of-art VCSELs in laboratories have been demonstrated to providemodulation bandwidths of 40 Gbps, current VCSEL technology makesachieving modulation bandwidths greater than 10 Gbps in practice verydifficult because of reduced device reliability if operated at the largecurrent densities required to do so. Therefore, VCSELs that can bereliably operated with greater modulation bandwidths are desired.

SUMMARY

The present invention provides a laser system having separatelyelectrically operable cavities for emitting modulated narrow linewidthlight with the system having a compound semiconductor material substratewith at least two first mirror pairs of semiconductor material layers ina first mirror structure on the substrate of a first conductivity typeand a first active region on that first mirror structure with pluralquantum well structures. There is at least twenty second mirror pairs ofsemiconductor material layers in a second mirror structure on the firstactive region of a second conductivity type and a second active regionon the second mirror structure with plural quantum well structures withat least two third mirror pairs of semiconductor material layers in athird mirror structure on the second active region of said firstconductivity type. An intermediate electrical interconnection isprovided at the second mirror structure and a pair of electricalinterconnections are provided separated by the substrate, the firstmirror structure, the first active region, the second mirror structure,the second active region, and the third mirror structure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows (a) a diagrammatic cross section view of a structurearrangement for a vertical cavity surface emitting laser, and (b) adiagrammatic cross section view of a structure arrangement for acomposite resonator vertical cavity laser,

FIG. 2 a shows a diagrammatic cross section view of a compositeresonator vertical cavity laser in a circuit arrangement providing forcombined current injection/electro-absorption operation, and FIG. 2 bshows a diagrammatic cross section view of a composite resonatorvertical cavity laser in a circuit arrangement providing for push-pulloperation,

FIG. 3 a shows a graph displaying plots of responses of a compositeresonator vertical cavity laser operated using current,electro-absorption and combined modulations with the electro-absorptionand current modulation in-phase, and FIG. 3 b shows a graph displayingplots of responses of a composite resonator vertical cavity laseroperated using current, electro-absorption and combined modulations withthe electro-absorption and current modulation out of phase,

FIG. 4 shows a graph displaying plots of responses of a compositeresonator vertical cavity laser operated using combined modulation withdifferent optical standing wave overlap ratios, ξ1/ξ2,

FIG. 5 shows graphs displaying plots of refractive index and normalizedoptical field intensity profiles for the short optical length cavity ofa composite resonator vertical cavity laser along the longitudinaldirection thereof if (a) the top cavity is shorter than the bottomcavity, and if (b) the top cavity is longer than the bottom cavity witheach graph having an inset thereby showing the optical field intensitynear the laser facet and the percentage of light at that facet,

FIG. 6 shows graphs displaying plots of responses of a compositeresonator vertical cavity laser operated using push-pull modulation for(a) different values of small signal cavities currents amplitudedifference A with currents phase difference θ=π, and for (b) differentvalues of small signal cavities currents phase difference θ with currentamplitude difference A=1,

FIG. 7 shows a graph displaying plots of responses of a compositeresonator vertical cavity laser operated using push-pull modulation withand without various device electrical equivalent parasitics, and plotsof responses of effectively a vertical cavity surface emitting laserusing standard current modulation with and without various deviceelectrical equivalent parasitics,

FIG. 8 a shows a graph displaying plots of responses of a verticalcavity surface emitting laser bonding pad interconnection cutofffrequency f_(c) resulting from interconnection parasitic capacitance asa function of the laser series resistance at the interconnection for aninterconnection effected with 50 μm bonding pads supported on implantedGaAs or on various dielectric thicknesses over implanted GaAs, and FIG.8 b shows a diagrammatic cross section view of a structure arrangementfor such interconnection alternatives,

FIG. 9 shows a diagrammatic cross section view of a structurearrangement for a composite resonator vertical cavity laser withassociated field amplitude distributions designated λ_(S) and λ_(L) asthe two longitudinal resonant modes of the combined cavities,

FIG. 10 shows a table setting out an epitaxial layer structure for acomposite resonator vertical cavity laser for emission at an examplewavelength,

FIG. 11 shows a table setting out typical material parameters for thedevice of FIG. 10,

FIG. 12 shows plots of long and short wavelength field amplitudedistributions versus relative cavity quantum well refractive indices,

FIG. 13 shows a graph displaying plots of output coupling efficienciesof long and short modes versus relative cavity quantum well refractiveindices,

FIG. 14 shows a graph displaying a plot of the extinction ratio ER of acomposite resonator vertical cavity laser between on-state and off-stateemitted light versus the number of periods of alternating optical indexmaterial layers in the bottom, intermediate and top mirrors therein,

FIG. 15 shows a table setting out typical device alternative parameterselections for a composite resonator vertical cavity laser,

FIG. 16 shows a table setting out further typical device parameters fora composite resonator vertical cavity laser,

FIG. 17 shows a graph displaying plots of various time domain responsesof a composite resonator vertical cavity laser with 44 intermediatemirror periods of alternating optical index material layers operatedwith I_(bias)=3.5 mA and I_(mod)=1.5 mA and modulated with a 20 Gbpssquare wave,

FIG. 18 shows a graph displaying plots of various time domain responsesof a vertical cavity surface emitting laser operated with I_(bias)=3.5mA and I_(mod)=1.5 mA and modulated with a 20 Gbps square wave,

FIG. 19 shows a graph displaying plots of various small signal responsesof a composite resonator vertical cavity laser with 50 intermediatemirror periods of alternating optical index material layers in solidlines, and plots of various small signal responses of a vertical cavitysurface emitting laser in dashed lines showing a gentler 10 dB/decaderoll-off for the composite resonator vertical cavity laser which alsohas a 3 dB bandwidth double that of the vertical cavity surface emittinglaser for the same operating electrical current,

FIG. 20 shows a graph displaying plots of various small signal responsesfor different modulations depths I_(mod)/I_(bias) of a compositeresonator vertical cavity laser with 50 intermediate mirror periods ofalternating optical index material layers, and a plot of the smallsignal response of a vertical cavity surface emitting laser that isindependent of modulation depth,

FIG. 21 shows a graph displaying plots of 3 dB modulation bandwidth as afunction of modulation depth of a composite resonator vertical cavitylaser with 50 intermediate mirror periods of alternating optical indexmaterial layers and a vertical cavity surface emitting laser,

FIG. 22 shows a diagrammatic cross section view of a composite resonatorvertical cavity laser in another circuit arrangement providing forpush-pull modulation and suitable operating bias current,

FIG. 23 shows a diagrammatic top view of a composite resonator verticalcavity laser structure showing major structural features and a“ground-signal-ground” contact layout, and

FIG. 24 shows an overlay of photolithographic mask layers used infabrication of a composite resonator vertical cavity laser structure.

DETAILED DESCRIPTION

A multiple resonant cavity vertical cavity surface emitting laser, 1,permitting very large rates of modulation in connection with thecoupling between those cavities, is shown on the right in (b) of FIG. 1in contrast to a conventional vertical cavity surface emitting laser(VCSEL), 2, shown on the left in (a) of FIG. 1. Multiple resonant cavityvertical cavity surface emitting laser 1, or composite resonatorvertical cavity laser (CRVCL) 1, in (b) of FIG. 1 has two outerdistributed Bragg reflector (DBR) mirrors, 3 and 4, (an upper outermirror 3 and a lower outer mirror 4) and two multi-quantum well (MQW)active regions, 5′ and 5″, separated by an intermediate DBR mirror, 6,between the two outer mirrors. MQW active regions 5′ and 5″ constitutethe p-n semiconductor junctions between upper mirror 3 and intermediatemirror 6, and bottom mirror 4 and intermediate mirror 6, respectively.The devices in FIG. 1 are formed as monolithic stacks of epitaxiallygrown layers that are subjected to subsequent fabrication process stepsin completing those structures. The entire CRVCL structure can be grownin a single growth layer sequence and fabricated in semiconductor wafersin much the same way as conventional VCSEL 2 is fabricated. In FIG. 1,CRVCL 1 in (b) on the right is a three-terminal device with a terminalon each of the outer ends including an upper terminal, 7, and a lowerterminal, 8, and an intermediate terminal, 9, on a mesa at the locationof intermediate mirror 6 to thereby allow each active region between themirrors to be independently biased or modulated, or both.

CRVCL 1 in this structure has the capability to change the photondensity therein by varying the gain or absorption in an upper cavity,10, comprising mirrors 3 and 6 along with active region 5′ therebetween,or in a lower cavity, 11, comprising mirrors 4 and 6 along with activeregion 5″ therebetween, while maintaining constant the current injectioninto the remaining other cavity (11 or 10, respectively), the furthercapability to detune an optical cavity by current injection, and the yetfurther capability to independently control carrier densities in bothcavities to thereby aid in achieving very large intensity modulationrates. That is, the coupled cavities 10 and 11 in CRVCL 1, underappropriate biasing conditions, lead to an increase in the small-signalbandwidth.

FIGS. 2( a) and 2(b) show a cross section of a slightly different CRVCLstructure again having upper mirror 3, lower mirror 4, and intermediatemirror 6 as DBR mirror stacks, and two resonant cavities 10 and 11containing the active regions 5′ and 5″, respectively, which can beindependently provided with injection current, i.e. “pumped,” or currentbiased or both. Two different modulation processes for a CRVCLstructure, or device, are described below in which voltage waveforms ofa voltage source as an input signal source are impressed on theintensity waveforms of the electromagnetic radiation emitted by thestructure during operation thereof. They are designated 1) simultaneouscurrent and electro-absorption modulation, or “combined modulation”illustrated in FIGS. 2( a), and 2) push-pull modulation, illustrated inFIG. 2( b). In each modulation process description, a small-signal modelbased on rate-equation analysis is presented. In addition, alarge-signal model is presented to show the large-signal response forthe push-pull modulation process.

FIGS. 2( a) and 2(b) also show an equivalent circuit model for how theCRVCL structure would effectively behave as an electrical circuitcomponent, and generally how such an effective circuit component wouldbe provided in an electrical circuit to arrange for the desired kind ofmodulation to be applied to that structure as indicated above for eachfigure. These figures assume that the CRVCL structure would be grownwith lower mirror 4 doped n-type, intermediate mirror 6 doped p-type,and upper mirror 3 doped n-type, thereby forming an n-p-n structure.However, the structure could also be grown p-n-p structure, with theupper and lower mirrors doped p-type, and the intermediate mirror dopedn-type. In this latter situation, the operation algebraic sign of theapplied voltages would be reversed, and the direction of current flowsin the structure would also be reversed.

The modulating current in CRVCLs of FIG. 2 can be introduced into one ofcavities 10 or 11 while the current injection into the remaining othercavity (11 or 10, respectively) is maintained constant, and this resultsin the modulation of the total carrier density, gain, and eventually theintensity of the electromagnetic radiation output of the laser. However,the magnitude of the relaxation oscillation frequency (ROF) due tointeractions of carriers and photons upon changes in the modulatingcurrent limits the large signal modulation although steps can be takento reduce this magnitude. Alternatively, means to decouple thisinteraction would lessen the relaxation oscillation effect to therebyprovide increased modulation bandwidth.

In the first variant of the present invention, one cavity is forwardbiased, so that current is injected (in FIG. 2( a) active region 5″ p-njunction in lower cavity 11 is shown forward biased by constant valuecurrent source I_(bias)). The modulated current density in theforward-biased active region 5″ p-n junction can be modulated eitherin-phase or out-of-phase with the electro-absorption (EA) voltagemodulation across the reverse biased remaining junction (shown in FIG.2( a) as active region 5′ p-n junction in upper cavity 11 being reversebias by constant value voltage source V1). Thus, in the device of FIG.2( a) we show the upper cavity active region 5′ p-n junction as reversebiased, and the lower cavity active region 5″ p-n junction as forwardbiased, but that device would function similarly if the lower cavityactive region 5″ p-n junction were reverse biased and upper cavityactive region 5′ p-n junction were forward biased. Variable voltagemodulation signal sources V1 _(mod), in series with voltage source V1,and V2 _(mod), in series with current source I_(bias), may or may nothave equal voltage values. The modulating voltage V2 _(mod) on theforward biased junction results in a modulation of the current flowI_(mod) through the junction such that the total currentI_(total)=I_(bias)+I_(mod).

The push-pull operating mode is illustrated in FIG. 2( b). In thisarrangement, where the resonator is balanced, large rate resonance-freemodulation is possible for a single longitudinal mode due tocarrier-induced index modulation of the output coupling efficiency. Inthis case both active region 5′ and 5″ p-n junctions are forward biased.A current source in each loop of the circuit maintains a constant totalcurrent in that loop, I_(bias)/2, and together, a constant total currentinto the device, I_(bias). A modulation voltage is added to one or theother junctions (in FIG. 2( b) shown to be in series with upper activeregion 5′ p-n junction) which results in a current modulation ofI_(mod). The result is a current flow of I_(bias)/2+I_(mod) in the upperactive region 5′ p-n junction, and a current modulation ofI_(bias)/2−I_(mod) in the lower active region 5″ p-n junction. In eithervariant illustrated by FIGS. 2( a) and 2(b), the relaxation oscillationbehavior can be significantly modified to result in greater bandwidth.

In analyzing the small-signal response of a CRVCL under the firstmodulation process, combined current and electro-absorption modulation,a modified rate-equation model is used with two carrier populations anda single longitudinal mode to describe the modulation response. Theassumption of a single longitudinal mode simplifies the rate equations,and is also appropriate for much of the operating range of the CRVCLsconsidered. Unlike the conventional modulation of a laser in which asmall-signal is introduced through current modulation, the CRVCLundergoes the modulation of absorption loss through application of areverse bias voltage to one of its cavities.

The rate equations for carrier and photon densities forcurrent/electro-absorption modulation are derived by assuming only oneoptical mode is lasing, or

$\begin{matrix}{\frac{\mathbb{d}N_{1}}{\mathbb{d}t} = {\frac{J}{qd} - \frac{N_{1}}{\tau_{1}} - {{vg}\;\xi_{1}S}}} & (1) \\{\frac{\mathbb{d}N_{2}}{\mathbb{d}t} = {{- \frac{N_{2}}{\tau_{2}}} + {\eta_{d}\;\xi_{2}S}}} & (2) \\{\frac{\mathbb{d}S}{\mathbb{d}t} = {{\left( {{\Gamma\;{vg}\;\xi_{1}} - \frac{\xi_{2}}{\tau_{p}}} \right)S} + {\beta\; R_{sp}}}} & (3)\end{matrix}$where N₁ and N₂ are the carrier densities in the two active regions (5′and 5″) (l/cm³), τ₁ and τ₂ are the carrier lifetimes in the currentmodulation cavity (10 or 11) and electro-absorption modulation cavity(11 or 10), respectively, J is the injection current density (A/cm²), qis the elementary charge (C), d is the gain region thickness (cm), ν isthe group velocity of the optical mode in the material (cm/s), g is thematerial gain (cm⁻¹), Γ is the optical confinement factor in theforward-biased cavity, τ_(p) is the photon lifetime, S is the photondensity (l/cm³), β is the spontaneous emission factor, and R_(Sp) is thespontaneous emission rate per unit volume (l/cm³ s). The quantities ξ₁and ξ₂ represent the fraction of the optical standing wave overlappingwith the current modulation cavity 10 or 11 and electro-absorptioncavity 11 or 10, respectively. The electro-absorption modulation cavity11 or 10 under reverse-bias behaves as a photodetector which convertsthe light emission from the current modulation cavity 10 or 11 into aphotocurrent in the electro-absorption modulation cavity 11 or 10. Thisprocess is accounted by assuming the photodetector efficiency η_(d) inequation (2).

As evident in equation (4), the total response of the device under bothcurrent and electro-absorption modulation can be considered as asuperposition of the response under conventional current modulation andelectro-absorption modulation separately. If the electro-absorptionmodulation is removed, i.e. m=0, the total response will become that ofa conventional laser under direct current modulation,

$\begin{matrix}{{\frac{s(\omega)}{S_{0}}} = {\frac{\Gamma\;\tau_{p}\omega_{r}^{2}{j(\omega)}}{{qd}\; S_{0}{D(\omega)}}}} & (6)\end{matrix}$On the other hand, if the current modulation is removed by settingj(ω)=0, then the total response will be in the same form as the laserresponse under the electro-absorption modulation,

$\begin{matrix}{{\frac{s(\omega)}{S_{0}}} = {{{\left( {\xi_{2}\frac{m}{\tau_{p}}\left( {{- {\mathbb{i}\omega}} + \frac{1}{\tau} + {{vg}^{\prime}S_{0}}} \right)} \right)/{D(\omega)}}}.}} & (7)\end{matrix}$

Comparing equations (6) and (7), the direct current modulation producesa relatively flat modulation response yet with somewhat limited 3-dBbandwidth, while the electro-absorption modulation produces theopposite. As illustrated in FIG. 3( a), combining both the modulationcomponents in-phase enables a new design freedom, so that a tradeoffbetween a flat response (a small relaxation oscillation peak) and highmodulation bandwidth can be made for an improved bandwidth. Moreover,another option is to combine both the modulation components out-of-phasesuch that the overall modulation bandwidth can be enhanced as shown inFIG. 3( b). This works better when the current and electro-absorptionmodulation are relatively flat by themselves. The electro-absorptionmodulation index m can be employed to tailor the combined modulationresponse. By combining current and electro-absorption modulationout-of-phase, a relatively flat response with a 3 dB modulationbandwidth of ˜90 GHz can be achieved (neglecting electrical parasitics).

Such a CRVCL allows detuning the cavity 10 and 11 lengths to permit eachcavity to have a different optical standing-wave overlaps ξ₁ and ξ₂.FIG. 4 illustrates the combined modulation response at a fixed photondensity S₀, while the relative overlap of the longitudinal mode witheach cavity is varied. As evident from the figure, the cavity detuningis another design freedom to produce an improved modulation response,indicating that a 100 GHz 3-dB bandwidth and only ˜1 dB relaxationoscillation peak can be achieved with an 80/20 distribution of thelongitudinal mode overlap between the two cavities. The currentinjection and voltage modulation create some of this detuning, but itcan also be built into the structure by growing the two cavities withdifferent cavity lengths, i.e. adjusting the thickness of active region5′ relative to active region 5″.

The second modulation process of a CRVCL, push-pull modulation, providesanother means to decouple the photon density and current density andminimize the relaxation oscillation effect. For push-pull modulation,the forward-bias injection current through both the top 10 and bottom 11cavities of a CRVCL will be modulated simultaneously but maintainedout-of-phase. As the carrier density increases in one cavity, thecarrier density in the other cavity decreases by an equal amount,maintaining a constant total carrier population. Hence, the net photonpopulation essentially decouples from changes in carrier population,which results in the elimination of the relaxation oscillation peak.

Without losing generality, we will analyze a CRVCL in which both theupper 10 (output) and lower 11 cavities are current modulated. This isan extension of the analysis for the current modulation in one cavity.Assuming that only one optical mode is lasing, the rate equations can bewritten as:

$\begin{matrix}{\frac{\mathbb{d}N_{1}}{\mathbb{d}t} = {\frac{J_{1}}{qd} - \frac{N_{1}}{\tau_{1}} - {{vg}_{1}\xi_{1}S}}} & (8) \\{\frac{\mathbb{d}N_{2}}{\mathbb{d}t} = {\frac{J_{2}}{qd} - \frac{N_{2}}{\tau_{2}} - {{vg}_{2}\xi_{2}S}}} & (9) \\{\frac{\mathbb{d}S}{\mathbb{d}t} = {{\left( {{\Gamma_{1}{vg}_{1}\xi_{1\;}} + {\Gamma_{2}{vg}_{2}\xi_{2\;}} - \frac{1}{\tau_{p}}} \right)S} + {\beta\; R_{sp}}}} & (10)\end{matrix}$where for cavity m, N_(m) is the carrier density (l/cm³), J_(m) is theinjection current density (A/cm²), τ_(m) is the carrier lifetime (s),g_(m) is the material gain (cm⁻¹), Γ_(m) is the optical confinementfactor, and the other parameters are defined the same as in equations(1) through (3).

The push-pull modulation response can be obtained by solving therate-equations (8) through (10), yielding

$\begin{matrix}\begin{matrix}\; \\{{\frac{s(\omega)}{S_{0}}} = {\frac{\begin{matrix}{{\Gamma_{1}{{vg}_{1}^{\prime}\left( {\frac{j_{1}(\omega)}{qd} - {{vg}_{10}S_{0}{\xi_{1}(\omega)}}} \right)}} +} \\{\Gamma_{2}{{vg}_{2}^{\prime}\left( {\frac{j_{2}(\omega)}{qd} - {{vg}_{20}S_{0}{\xi_{2}(\omega)}}} \right)}}\end{matrix} \cdot \frac{{{\mathbb{i}}\;\omega} - \left( {\frac{1}{\tau_{1}} + {{vg}_{1}^{\prime}\xi_{10}S_{0}}} \right)}{{{\mathbb{i}}\;\omega} - \left( {\frac{1}{\tau_{2}} + {{vg}_{2}^{\prime}\xi_{20}S_{0}}} \right)}}{\begin{matrix}{\omega^{2} + {{\mathbb{i}}\;\omega\left( {\frac{1}{\tau_{1}} + {{vg}_{1}^{\prime}\xi_{10}S_{0}}} \right)} -} \\{{v^{2}\Gamma_{1}\xi_{10}g_{10}g_{1}^{\prime}S_{0}} - {v^{2}\Gamma_{2}\xi_{20}g_{20}g_{2}^{\prime}S_{0}}}\end{matrix} \cdot \frac{{{\mathbb{i}}\;\omega} - \left( {\frac{1}{\tau_{1}} + {{vg}_{1}^{\prime}\xi_{10}S_{0}}} \right)}{{{\mathbb{i}}\;\omega} - \left( {\frac{1}{\tau_{2}} + {{vg}_{2}^{\prime}\xi_{20}S_{0}}} \right)}}}}\end{matrix} & (11)\end{matrix}$where for cavity m, g_(m0) is the steady-state material gain (l/cm),g_(m)′ is the differential gain (l/cm²), and ξ_(m0) is the percentage ofstanding-wave overlap under steady-state. However, by assuming identicalcavity conditions, equation (11) can be simplified to:

$\begin{matrix}{{{H(\omega)}} = {{\frac{s(\omega)}{j_{1}(\omega)}} = {\frac{\frac{\omega_{r}^{2}{\Gamma\tau}_{p}}{qd} \cdot \left( {1 + {A\;{\mathbb{e}}^{j\;\theta}}} \right)}{\left\lbrack {\left( {\omega^{2} - \omega_{r}^{2}} \right)^{2} + {\omega^{2}\gamma^{2}}} \right\rbrack^{1/2}}}}} & (12)\end{matrix}$where A and θ account for the phase and amplitude difference between thesmall-signal current components such thatj ₂(ω)=A·J ₁(ω)·e ^(jθ)  (13).

The response given by the equation (12) is similar to the modulationresponse of a conventional laser, with additional dependence on A and θ.

In push-pull modulation, the external observability of the modulationresponse of the two cavities must be determined, i.e. whether externallight modulation occurs which can be used to carry information.Therefore push-pull modulation requires additional analysis tounderstand the effects on the longitudinal modes.

Longitudinal mode modulation arises from the dynamic cavity detuningunder the (differential) current injection through both cavities 10 and11, leading to a change in the relative coupling efficiencies of the top3 and bottom 4 mirrors. Under the condition of A=1 and θ=π, in push-pullmodulation, the carrier density always increases in one cavity 10 or 11while it decreases in the remaining other cavity 11 or 10, respectively.Owing to the carrier-induced index change, the effective length for onecavity will decrease, while for the other cavity the length willincrease. As a result, the optical mode will be either “pushed” towardsthe CRVCL substrate 12 producing less output light, or “pulled” towardsthe output facet 13 producing more output light as indicated in FIG. 5.All this occurs while the photon lifetime and hence the total photonpopulation remains unchanged, i.e. the total number of photons exitingthe cavity through the top 3 and bottom 4 mirrors remains constant.Since the longitudinal mode modulation is not natively included in therate equation analysis, it has to be accounted for separately.

The longitudinal mode modulation is assumed to be 10% of the availablelight at the output facet. This assumption will be justified later. Theequation (12) is modified to account for the longitudinal modemodulation by adding an additional term:

$\begin{matrix}{{{s_{out}(\omega)}} = {{{{\frac{{j_{1}(\omega)} \cdot \frac{\omega_{r}^{2}{\Gamma\tau}_{p}}{qd} \cdot \left( {1 + {A\;{\mathbb{e}}^{{\mathbb{i}}\;\theta}}} \right)}{\left\lbrack {\left( {\omega^{2} - \omega_{r}^{2}} \right)^{2} + {\omega^{2}\gamma^{2}}} \right\rbrack^{1/2}} \cdot 0.15}\%} + {10{\% \cdot 0.15}{\% \cdot S_{0}}}}}} & (14)\end{matrix}$where s_(out)(ω) is the small-signal photon density at the laser facet.The factor 0.15% represents the percentage of the total internalintensity present at the output laser facet under steady-stateconditions.

FIG. 6 is plotted to illustrate the frequency dependence of thepush-pull modulation response and the dependence thereof on A and θ. Asindicated in the figure, an extraordinarily wide and flat modulationbandwidth (>80 GHz) is possible under the push-pull modulation scheme,and the behavior is markedly different than the equivalent structureunder “normal” direct current modulation. The most pronounced feature isthat the relaxation oscillation vanishes for A=1 and θ=π (black curves),which corresponds to the small-signal current for both cavities havingthe same amplitude and being exactly out of phase. The modulationbandwidth is therefore limited only by the photon lifetime, ifelectrical parasitic effects are neglected. As can also be seen in thefigure, the modulation response is reasonably tolerant to deviationsfrom the ideal push-pull modulation condition A=1 and θ=π, which makesthe operation of the device tolerant to manufacturing variations.

The calculations so far assume the identical top 10 and bottom 11optical cavities. However, it can be expected that this may be difficultto achieve in practice. For example, different current densities J₁₀ andJ₂₀ would likely result in different values of ξ₁/ξ₂, g₁′/g₂′ andg₁₀/g₂₀. Therefore, the full form of the modulation response given byequation (11) was invoked with the inclusion of the longitudinal modemodulation terms. It was found that the modulation response digressesfrom the ideal condition only slightly for a relatively large change inξ₁/ξ₂, suggesting that a substantial fabrication tolerance exists.

Currently, intrinsic laser bandwidth and reliability at high currentdensities have been the modulation rate limiting factors in commerciallyavailable VCSEL-based transceivers, which generally operate at 10 Gb/sand below. Accordingly, at such frequencies, there is little need to beconcerned about minimization of electrical parasitics affecting devicesused therein. Of course, such parasitics are a fact of life and cannotsimply be ignored, certainly not at larger frequencies. FIG. 7illustrates the modeled push-pull modulation response of a CRVCL whenboth the upper 10 and lower 11 cavities are limited to 20 GHz electricalbandwidth, typically encountered in commercial devices, due to suchparasitics as diode forward resistance, diode junction capacitance, andinterconnection capacitances. The electrical parasitics introduce adominant pole to the push-pull modulation response, and thus dominatethe overall modulation bandwidth.

Therefore, the improvements in intrinsic laser bandwidth afforded bypush-pull modulation of the coupled cavities in a CRVCL device can onlybe taken advantage of in conjunction with a reduction in deviceparasitics. By far, the dominant parasitic element in VCSELs is thedevice contact bonding pad capacitance C_(bp), which together with thedifferential series resistance R_(d) of the diode form a low pass filterwith a cutoff frequency f_(c)=½πR_(d)C_(bp). The contact bonding pad isthe metal area connected to terminals 7 and 9 which allow the device tobe connected to the outside world, such as a driver IC, by a bond wireor solder bump between the bonding pad and the IC. A capacitance isassociated with the interface between this bonding pad metal and thesemiconductor underneath. There is definitely something to be gainedfrom minimizing the series resistance, but given the current advancedstate-of-the-art in DBR design, it is probably not realistic to expectresults significantly better than what is commonly achieved today. Thisis to say that R_(d) will almost certainly be in the range of 50 to 100ohms for the 5 to 10 micron oxide aperture devices envisioned.

FIG. 8( a) shows a plot of f_(c) vs R_(d) for the structure illustratedin FIG. 8( b). The structure in FIG. 8( b) shows a bonding pad 14 whichhas a linear dimension of 50 μm, on top of a 1- to 3-μm thick Cyclotene™dielectric layer 15, which in turn is on top of GaAs 16 which has animplanted region 17 that extends to a depth of 3 μm from the top surfaceof the GaAs. 50 μm is used as it is the smallest diameter bonding padthat can practically be used for a high manufacturing yield. As can beseen in FIG. 8( a), the incremental gain in going from 80 ohms to 60ohms is relatively minor when compared to the advantage of putting thepad metal on thick dielectric. This is as opposed to putting it directlyon the implanted GaAs, as is commonly done. The data shows that even forthe limiting case of a 100-ohm device, f_(c)>70 GHz can be obtained byputting the bonding pads on as little as 2 μm of Cyclotene(BCB).

The foregoing analyses have illustrated the small-signal modulationresponse of the CRVCL. However, use in communication systems requiresmodulation using large amplitude signals. Therefore, we will describespecific structures, and the large-signal response of these structures.FIG. 9 shows a schematic cross-section of the resonant structure of thecoupled cavities in a suitable CRVCL, and FIG. 10 shows the detailedstructure for a device as implemented to emit at a wavelength of 850 nm.However, the CRVCL structure can be implemented to emit at anywavelength that a more conventional structure emits at, ranging from UV(350 nm) to the IR (2500 nm). In the following description we assume thenotation Al_(x)Ga_(1-x)As where x defines the relative content ofaluminum to gallium in any given layer. For instance, x=1 refers to AlAsand x=0 refers to GaAs. For simplicity, we will drop the subscripts inthe following description and refer to the value of x to describe thecomposition. We will also refer to some thicknesses in terms of opticalthicknesses λ of the emission wavelength. For instance, for a deviceemitting at 850 nm, a 1λ optical thickness cavity in AlGaAs materialswill be approximately 235 nm in physical thickness, since the refractiveindex n is approximately 3.6.

As shown in FIGS. 9 and 10, the resonator consists of a lower DBR stack4 with Nb periods of alternating low index 18 and high index 19quarter-wavelength thick layers with composition graded layers 20between the low and high index layers, an intermediate central DBR 6with Nc periods, and an upper top DBR 3 with Nt periods. The high index19 and low index 18 materials in the DBRs consist of x=0.15 and x=0.95to 1.0 AlGaAs respectively. These compositions are chosen to provide themaximum refractive index contrast, while avoiding compositions thatwould absorb the emitted light. Clearly, other compositions could bechosen as well. The nominal x=0.15 composition could range from x=0.05to x=0.30, while the nominal x=0.95 AlGaAs could range from x=0.80 to0.97 for an oxide current aperture, or to x=1.0 for current confinementprovided by proton implantation or the etching of a mesa. For emissionat other wavelengths, similar considerations would guide the choice ofcomposition for the mirrors. Linearly graded interfaces 20 are used toreduce electrical series resistance. The grade could range tothicknesses as large as 200 nm.

At the interfaces between the mirrors are two active regions 5′ and 5″with each cavity total optical thickness equal to 1λ when cold, eachcontaining two symmetric 65% AlGaAs spacer layers 21 and five 7 nm thickGaAs quantum wells 22 with eight barriers with x=0.25 separating andsurrounding the quantum wells 23. The number of quantum wells could beas few as one, and as many as 7. Quantum well thickness could also varyfrom 5 nm to 10 nm, and the barrier layer thickness could also vary.Space layer compositions could vary from 30% to 70%. The substrate 12 isformed of GaAs. FIG. 11 lists the material parameters.

Some additional layers are included in the structure to provide controlover the current flow through the device. Included in the intermediatemirror 6 are two regions designated as oxidation structures 24. Thex=0.98 layer 32 is partially oxidixed later in the process to confinethe current to the center of the device. Other layers around the x=0.98layer are lower Al content x=0.97 (33) and x=0.65 (34) and are designedto help control the thickness of the oxidation layer. Contact layers 25at the top surface include a GaAs and AlGaAs layer with x=0.15 that areheavily doped to provide a low resistance contact to metals deposited onthe top surface. The stop etch layer 26 and contact layer 27 are used toprovide the metal contact for terminal 9 that contacts the intermediatemirror. The stop etch layer 26 is designed to be very slow etching sothat a mesa can be etched down to this layer and stopped accurately. Thecontact layer 27 below it is more heavily doped and lower aluminumcontent so that we can make a low resistance contact to the structure.

Being a coupled-cavity resonator, the structure of FIG. 9 supports twolongitudinal resonant modes, denoted λ_(L) and λ_(S), where λ_(L)=λ₀+Δλand λ_(S)=λ₀−Δλ. The splitting of the resonant wavelengths is strictlydetermined by the number of periods N_(c) in the intermediate mirror 6.As N_(c) is increased, the two cavities 10 and 11 become increasinglydecoupled and the two modes tend towards degeneracy.

As shown in FIG. 12, each mode is further characterized by its electricfield distribution within the composite cavity. Characteristic of thistype of structure, asymmetry of the field profiles is only manifest inthe presence of a difference in the optical path lengths of the MQWactive regions 5′ and 5″, regardless of the degree of coupling betweenthe two cavities. For identical cavities, the field profiles aresymmetric and degenerate for the two modes, i.e. the spatialdistributions are equally split between the two cavities. As the ratioof the relative optical thicknesses of the two active regions 5′ and 5″departs from unity, the peak modal e-field intensities migrate to one orthe other cavity.

Under cold (unpumped) conditions, the two optical cavities 10 and 11 areidentical by design Δn=0 in the figure). Any change in the relativerefractive indices of the quantum wells 22 (Δn=n_(qw1)−n_(qw2)) willcause the relative optical thicknesses to change. In addition, the longand short mode field profiles behave in opposite respect. For example,as the optical path length of the active layer 5″ is decreased withrespect to that of the upper active layer 5′, the peak field intensityof the short wavelength mode shifts to the lower cavity 11 while that ofthe long wavelength mode shifts to the upper cavity 10. As the modalfield distributions change, so too does the relative efficiency withwhich the individual modes couple out of the resonator through eitherthe top 3 or bottom 4 mirror as indicated in FIG. 13. This figure showsthe ratio of the coupling out of the top surface of the resonator to thebottom surface of the resonator for both the long and short wavelengthresonator modes, λ_(L) and λ_(S), respectively. This relative change incoupling efficiency is what is exploited to impart amplitude modulation.

The physical mechanism that changes the optical path lengths in the twoactive regions 5′ and 5″ is the change in refractive index of the GaAsquantum wells 22 with injected carrier density. This is the samemechanism that gives rise to transient and adiabatic chirp in allsemiconductor lasers, a dynamic shift in laser frequency undermodulation which ceases as the carrier density in the laser cavityreaches equilibrium. The magnitude of the effect is relatively small,with dn/dN being on the order of 1.2×10⁻²⁰ cm⁻³. Given that the quantumwells 22 comprise approximately 15% of the 1λ active region 5′ or 5″,this implies that a ˜0.05% shift in the optical thickness of the spacersis achievable for a ΔN of ˜1e18 cm⁻³ (a reasonable number). While small,this degree of cavity shift is indeed adequate to achieve a significant(>3 dB) modulation in the output power of the device as is seen in thefollowing.

The dynamic state of this structure is modeled using rate equations forthe two carrier populations (upper 10 and lower 11 cavity) and twophoton populations (long and short mode). The rate equations are set outin somewhat unconventional form in being in terms of the carrierdensities N₁ and N₂ in the two active regions 5′ and 5″, and the totalphoton numbers S_(S) and S_(L) in the two optical modes. This isbecause, in this instance, the concept of expressing the mode equationsin terms of photon densities using an equivalent mode volume is noteasily accomplished due to the difficulty in defining mode volume inconnection with photons. The four coupled differential equations are:

$\begin{matrix}{\frac{\mathbb{d}N_{1}}{\mathbb{d}t} = {\frac{\eta_{i}I_{1}}{{qV}_{1}} - \frac{N_{1}}{\tau_{e}} - {BN}_{1}^{2} - {\frac{v_{g}g_{0}}{\chi_{1}}{\ln\left( \frac{N_{1} + N_{0}}{N_{tr} + N_{0}} \right)}\frac{\left( {{\Gamma_{1}^{S}S_{S}} + {\Gamma_{1}^{L}S_{L}}} \right)}{V_{1}}}}} & (16) \\{\frac{\mathbb{d}N_{2}}{\mathbb{d}t} = {\frac{\eta_{i}I_{2}}{{qV}_{2}} - \frac{N_{2}}{\tau_{e}} - {BN}_{2}^{2} - {\frac{v_{g}g_{0}}{\chi_{2}}{\ln\left( \frac{N_{2} + N_{0}}{N_{tr} + N_{0}} \right)}\frac{\left( {{\Gamma_{2}^{S}S_{S}} + {\Gamma_{2}^{L}S_{L}}} \right)}{V_{2}}}}} & (17) \\{\frac{\mathbb{d}S_{S}}{\mathbb{d}t} = {{v_{g}{g_{0}\left\lbrack {{{\ln\left( \frac{N_{1} + N_{0}}{N_{tr} + N_{0}} \right)}\frac{\Gamma_{1}^{S}}{\chi_{1}}} + {{\ln\left( \frac{N_{2} + N_{0}}{N_{tr} + N_{0}} \right)}\frac{\Gamma_{2}^{S}}{\chi_{2}}}} \right\rbrack}S_{S}} - \frac{S_{S}}{\tau_{pS}} + {\frac{\beta}{2\tau_{e}}\left( {{V_{1}N_{1}} + {V_{2}N_{2}}} \right)}}} & (18) \\{\frac{\mathbb{d}S_{L}}{\mathbb{d}t} = {{v_{g}{g_{0}\left\lbrack {{{\ln\left( \frac{N_{1} + N_{0}}{N_{tr} + N_{0}} \right)}\frac{\Gamma_{1}^{L}}{\chi_{1}}} + {{\ln\left( \frac{N_{2} + N_{0}}{N_{tr} + N_{0}} \right)}\frac{\Gamma_{2}^{L}}{\chi_{2}}}} \right\rbrack}S_{L}} - \frac{S_{L}}{\tau_{pL}} + {\frac{\beta}{2\tau_{e}}\left( {{V_{1}N_{1}} + {V_{2}N_{2}}} \right)}}} & (19)\end{matrix}$where,

N₁, N₂: carrier densities in first 5′ and second 5″ active regions inl/cm³

S_(S), S_(L): photon number in short and long wavelength modes

I₁, I₂: current injection in first 5′ and second 5″ active regions inamps

V₁, V₂: volume of first 5′ and second 5″ active region in cm³

B: bimolecular recombination coefficient in cm³/s

β: spontaneous emission factor

g₀: gain coefficient in l/cm³

N_(tr): transparency carrier density in l/cm³

N₀: gain fitting parameter in l/cm³

τ_(e): spontaneous carrier lifetime in seconds

τ_(pL), τ_(pS): photon lifetimes of long and short wavelength modes inseconds

V_(g): group velocity in cm/s

η_(i): internal quantum efficiency

q: elementary charge in Coulombs

The Γ's are the 2×2 matrix of confinement factors of the two modes withthe two active regions 5′ and 5″,

$\begin{matrix}{\Gamma_{1,2}^{S,L} = {\frac{\int_{{V\; 1},{V\; 2}}{E_{S,L}^{*}E_{S,L}\ {\mathbb{d}V}}}{\int_{- \infty}^{+ \infty}{{E_{S,L}}^{2}\ {\mathbb{d}V}}}.}} & (20)\end{matrix}$For the gain function, we have used the logarithmic expression

$\begin{matrix}{g = {g_{0}{{\ln\left( \frac{N_{1} + N_{0}}{N_{tr} + N_{0}} \right)}.}}} & (21)\end{matrix}$Here, g₀ is the gain coefficient in cm⁻¹, N_(tr) is the transparencycarrier concentration, and N₀ is a fitting parameter to account forabsorption under low injection N<<N_(tr). The terms χ₁ and χ₂ accountfor the gain suppression due to photon saturation

$\begin{matrix}{\chi_{i}\sqrt{1 + \frac{\left( {{\Gamma_{i}^{S}S_{S}} + {\Gamma_{i}^{L}S_{L}}} \right)}{V_{i}P_{sat}}}} & (22)\end{matrix}$where P_(sat) is the photon saturation density in cm⁻³.

The total power emitted, taken from the top mirror 3 is written as thesum of the power emitted from the short and long wavelength modes

$\begin{matrix}{{P = {P_{S} + P_{L}}}{where}} & (23) \\{P_{S} = {{\frac{{hv}\;\eta_{ext}}{\tau_{pS}}\frac{\theta_{S}}{\theta_{S} + 1}S_{S}\mspace{14mu}{and}\mspace{14mu} P_{L}} = {\frac{{hv}\;\eta_{ext}}{\tau_{pL}}\frac{\theta_{L}}{\theta_{L} + 1}{S_{L}.}}}} & (24)\end{matrix}$Here, η_(t) is the external quantum efficiency α_(m)/(α_(m)+α_(i)).θ_(S) and θ_(L) are functions that describe the relative power splittingbetween the top 3 and bottom 4 mirror for each mode

$\begin{matrix}{\theta = {\frac{P_{top}}{P_{bottom}}.}} & (25)\end{matrix}$In all of the above, the Γ's, θ's, and τ_(p)'s are all dynamic functionsof the carrier densities N₁ and N₂.

Solving equations (16) through (19) and (23) in the time domain requiresexpressions as functions of the carrier densities N₁ and N₂ for theconfinement factors Γ₁ and Γ₂, the output coupling functions θ_(S) andθ_(L), and finally for the photon lifetimes τ_(pS) and τ_(pL). Ratherthan resorting to a first-principles analytical treatment for thedetermination of these functional relationships, a behavioral approachis followed in which the parameters for the structure of FIG. 9 arefirst calculated using the parameters of FIG. 11, and then they arefitted with an appropriate functional form.

Associated with each mode is a photon lifetime, dependent upon thedistributed mirror losses due to material absorption (α_(i)) andtransmission through the mirrors (α_(m)):

$\begin{matrix}{\tau_{p} = \frac{1}{v_{g}\left( {\alpha_{i} + \alpha_{m}} \right)}} & (26)\end{matrix}$where v_(g) is the group velocity of the mode in question. Thedistributed mirror loss can be further separated into contributions fromthe top 3 and bottom 4 mirrorsα_(m)=α_(m) ^(t)+α_(m) ^(t)  (27a).The emission from the device as observed through the top mirror 3 willbe proportional to the ratio of the mirror losses

$\begin{matrix}{\theta \propto \frac{\alpha_{m}^{t}}{\alpha_{m}^{b}}} & \left( {27b} \right)\end{matrix}$Requiring thatα_(m)=α_(m) ^(t)+α_(m) ^(b)=constant  (28)will assure that the modal threshold gain will remain constant, as τ_(p)is constant as well. By keeping the modal threshold gain constant, thecarrier and photon populations will undergo a minimum perturbation asthe mode “sloshes back and forth” between the upper 10 and lower 11cavities.

The dependence of dτ/dn, the derivative of the photon lifetime with theindex difference between the quantum wells in the two active regionsn_(qw1)-n_(qw2) on the values of Nt, Nb and Nc needs to be considered.The results show that for Nb−Nt=3, the photon lifetime is invariant withΔn under all DBR combinations.

This result merely states that, for τ_(p) to remain constant, thereflectivities of the upper 3 and lower 4 DBRs should be the same. Thedifference of 3 periods accounts for the fact that the upper DBR 3 isterminated in air while the lower DBR 4 is terminated in GaAs 12.Therefore, for the remainder of the modeling exercise, we set Nb−Nt=3,so that,τ_(pL)=τ_(pS)=τ_(p)  (29).

The output coupling coefficients θ_(S)(N₁,N₂) and θ_(L)(N₁,N₂) arecalculated as the ratios of the Poynting vector magnitudes of the modale-fields taken at the top 3 and bottom 4 mirrors

$\begin{matrix}{\theta = {\frac{S_{top}}{S_{bottom}}.}} & (30)\end{matrix}$The long and short wavelength modes are found to be symmetric in Δn suchthat θ_(L)(Δn)=θ_(S)(−Δn).

These output coupling coefficients have been found to be quite wellapproximated by an exponential for small Δn, and so the functions forthe coupling coefficients have been chosen as

$\begin{matrix}{\theta_{S} = {{\theta\left( {\Delta\; n} \right)} = {{K\;{\mathbb{e}}^{Q\;\Delta\; n}} = {K\;{\mathbb{e}}^{Q\frac{\mathbb{d}n}{\mathbb{d}N}{({N_{1} - N_{2}})}}}}}} & (31) \\{\theta_{L} = {{\theta\left( {{- \Delta}\; n} \right)} = {{K\;{\mathbb{e}}^{{- Q}\;\Delta\; n}} = {K\;{{\mathbb{e}}^{Q\frac{\mathbb{d}n}{\mathbb{d}N}{({N_{2} - N_{1}})}}.}}}}} & (32)\end{matrix}$Equations (31) and (32) are substituted into (24) for the numerical timedomain simulation. For each different value of Nc examined, the valuesfor K and Q can be extracted. The values for K and Q are tabulated inFIG. 15.

The maximum power deviation coupled through the top mirror 3 of thedevice through aperture 13 under modulation can be expressed in the formof an extinction ratioER=θ(Δn)/θ(−Δn)  (33).This extinction ratio is an important parameter for communicationapplications, as one wants a large contrast between the amount of lightemitted from the device in the on-state and the off-state. Communicationstandards will often specify a minimum acceptable extinction ratio. FIG.14 represents the extinction ratio as a function of Nc. Nt is variedfrom 14 to 30, Nb from 17 to 33, and Nc from 0 to 33. The vertical barsthat become more prominent at higher values of Nc corresponds to therange of Nt and Nb that are assumed in the simulation. This plotdemonstrates that the extinction ratio is essentially determined solelyby the number of intermediate DBR 6 pairs Nc, with very littledependence upon Nt or Nb. Therefore, for a given Δn, a larger value ofNc will yield an increase in modulation depth.The confinement factorsΓ₁ ^(S)=Γ₂ ^(L)=Γ₁; Γ₁ ^(L)=Γ₂ ^(S)=Γ₂  (34)can be calculated using equation (20) for various values of Nc, againwith Nb−Nt=3. We find that for small Δn, the confinement factors Γ₁ andΓ₂ are reasonably well approximated as linear functions of Δn, and soexpress them as

$\begin{matrix}{\Gamma_{1} = {{\Gamma_{0} - {\frac{\mathbb{d}\Gamma}{\mathbb{d}n}\left( {n_{{qw}\; 1} - n_{q\; w\; 2}} \right)}} = {\Gamma_{0} - {\frac{\mathbb{d}\Gamma}{\mathbb{d}n}\frac{\mathbb{d}n}{\mathbb{d}N}\left( {N_{1} - N_{2}} \right)}}}} & (35) \\{\Gamma_{2} = {{\Gamma_{0} + {\frac{\mathbb{d}\Gamma}{\mathbb{d}n}\left( {n_{{qw}\; 1} - n_{q\; w\; 2}} \right)}} = {\Gamma_{0} + {\frac{\mathbb{d}\Gamma}{\mathbb{d}n}\frac{\mathbb{d}n}{\mathbb{d}N}\left( {N_{1} - N_{2}} \right)}}}} & (36)\end{matrix}$Equations (35) and (36) are substituted into equations (16) through (19)for the numerical time domain simulation. For each different value of Ncexamined, we extracted values for Γ₀ and dΓ/dn.

Performance was evaluated for numerous cases as Nc was varied from 25 to50, all with Nt=17 and Nb=20. The values of Nc correspond to anintermediate mirror reflectivity of greater than 99%. FIG. 15 lists thedesign choices examined, along with the constants used in the behavioralmodels developed in the previous section. A 5 μm active region aperture13 is used, although the device concept tolerates a range of apertures.

Performance was evaluated by the measurement of eye diagrams usingsymmetrical current modulation (i.e. I₁=I_(bias)+/−I_(mod) andI₂=I_(bias)−/+I_(mod)) in each cavity and long pattern lengthpseudorandom pulse sequences. An eye diagram consists of overlapping thesignals from a long string of random “1”s and “0”s. An example is shownin the inset picture in FIG. 17. One can see the “1” and “0” levels, aswell as the lines corresponding to the transitions between the twostates. A high quality signal has a large open area between the twosignal levels and the transitions. This corresponds to a condition wherethere will be few errors transmitting the signal. Along with FIG. 15,other parameters assumed in the rate equations analysis are listed inFIG. 16.

FIG. 17 shows the time domain response of a CRVCL with Nc=44 operatedwith I_(bias)=3.5 mA and I_(mod)=1.5 mA modulated with a 20 Gbps squarewave. Plotted in the figure are the carrier densities N₁ and N₂, alongwith their average carrier density N_(ave). Emitted optical poweroriginating from the long and short wavelength mode is also shown. Thelong-wavelength mode power is multiplied by a factor of 100 for clarity.In the inset is the eye diagram generated with the pseudorandom bitsequence.

After the initial turn-on transient, the response settles down as in atypical laser response. As expected, the modulation present in thecarrier densities is small, with the average being almost flat. Theoutput power is entirely dominated by the short wavelength mode. Themodulation of the long mode's confinement factor, and hence itsthreshold gain, is 180 degrees out of phase with the current modulationin the two active regions. Hence, the long wavelength mode is almostcompletely suppressed and never reaches threshold.

As shown in the insert, the eye diagram is very clean. There is acomplete lack of deterministic jitter and overshoot, in spite of therelatively large extinction ratio and low I_(bias) the average currentthrough the device. Note that the rise and fall characteristics look fardifferent from those of a typical semiconductor laser.

In contrast, FIG. 18 shows the response of a conventional VCSEL usingthe same physical parameters as in FIG. 17. Here, the same biasconditions and photon lifetime were used, but the confinement factor wasmultiplied by 2 to account for the single active region.

In this case, there is a much larger modulation of the carrier density,and there is severe eye diagram closure due to deterministic jitter andovershoot. Certainly, the eye diagram could be improved by increasingthe bias current to unrealistic levels, but, when driven at the samecurrent density, the CRVCL has far superior modulation characteristics.

We have found that Nc needs to be larger than 20 periods, orequivalently, the intermediate mirror reflectivity needs to be greaterthan 99%, for a minimum 2 dB extinction ratio. For other emissionwavelengths and materials systems, the intermediate mirror reflectivityrequirement of greater than 99% for good extinction ratio remains thesame. However, this requirement may result in a different number ofintermediate mirror pairs, depending upon the refractive indexdifferences of the materials used in the mirror.

Small signal scattering parameter S₂₁ response characteristics wereexamined for a CRVCL with Nc=50 and a conventional VCSEL with the samematerial parameters, aperture size, and photon lifetime. S₂₁ is aforward transmission coefficient, and describes the ratio of an outputsignal (optical in this case) to an input signal (electrical in thiscase) for the CRVCL. The S₂₁ curves for the CRVCL are the four solidlines, and the S₂₁ signal curves for the conventional VCSEL are thedotted lines. FIG. 19 shows the results for bias currents of 2 to 5 mAand 0.1 mA modulation depth for both types of devices. For the twostructures, the relaxation oscillation frequency is essentially thesame, but the CRVCL shows much lower peaking and a gentler roll-off of10 dB/decade, leading to a substantially improved 3 dB bandwidth. Thisresult is consistent with the small signal analysis provided above.

The CRVCL 3 dB bandwidth is shown to improve with modulation depth. FIG.20 shows the calculated small signal S₂₁ response for a CRVCL (Nc=50)for different values of modulation depth I_(mod)/I_(bias). For amodulation depth of 20%, the 3 dB frequency exceeds 80 GHz. This is incontrast to the more typical behavior of the conventional VCSEL (alsoshown) which is independent of modulation depth as shown in FIG. 21.When compared to the conventional VCSEL, there is a factor of fourincrease in modulation bandwidth at a given current density.

FIG. 22 shows a more detailed cross section view of an example of theCRVCL of the present invention. The device includes two 5QW 22 1λ,active regions 5′ and 5″, dual oxide apertures 29′ and 29″ for currentconfinement in the upper and lower cavities, a highly doped intracavitycontact layer 27, and a GaInP 26 etch stop for the intracavity contact 9etch.

Also shown in the figure is a bias and modulation arrangement examplefor the push-pull mode of operation. Bias is applied via a currentsource to the intracavity contact 9, whereas the modulating voltage isapplied to the upper cavity via a topside contact 7. As the devicebehaves in a circuit sense essentially as a back-to-back diode,modulating the device in such fashion directly leads to the desireddifferential modulation of the cavity currents. This arrangement allowsfor the minimization of the bonding pad size, and avoids any parasiticswhich might otherwise be a factor should modulation have been appliedvia the intracavity contact. In any case, minimizing stray parasitics isimportant.

As shown in FIG. 23, which is a view of the device from the topside ofthe chip, the example device employs a “bow tie” contact structure. Theupper terminal 7 metal contact comes from one side of the device, whilethe intermediate terminal 9 metal contact comes to the other side of thedevice. A ground 29-signal 30-ground 29 bond pad arrangement is used toenable wirebonding or on-chip probing, or both, using coplanar waveguideprobes. The ground 29 bond pads are attached to the lower terminal 9,while the signal bond pad 30 is attached to the upper terminal 7 metal.The ground-signal-ground (G-S-G) is a standard arrangement for theapplication of very high frequency signals.

The epitaxial structure in the device example contains 17 to 20 top 3n-doped DBR pairs, 20 to 23 bottom 4 n-doped DBR pairs, and 38 to 50intermediate 6 p-doped DBR pairs. All DBRs consist of x=0.15/x=0.95AlGaAs layers 18, 19 with 20 nm linear compositional and doping grades20 to reduce the series resistance. The p DBRs and n DBRs are doped 3e18cm⁻³ and ˜2e18 cm⁻³ respectively, with the doping in the 6 pairsadjacent to the cavities reduced by ˜50% to reduce free carrierabsorption. The 5 central mirror pairs in the low field region of theintermediate DBR are highly doped (p>5e18 cm⁻³) for improved currentspreading, with one pair doped >2e19 cm⁻³ to facilitate the Ti/Auunalloyed ohmic contact 9, which is made to a 12 nm thick GaAs layer 27placed at a standing wave node to minimize absorption. Adjacent to andimmediately above the contact layer is a 10 nm thick GaInP layer 26 foruse as an etch stop during the intracavity contact etch. A 15 nm thickGaAs ohmic contact layer doped n>>8e18 cm⁻³ caps off the structure 25.

The active regions 5′ and 5″ each have five undoped 7 nm thick GaAsquantum wells 22 with 6 nm thick barriers 23 of undoped x=0.25 AlGaAsbetween the quantum wells and 20 nm of the barrier material on eitherside of the quantum well region. The remaining cavity spacer material isof low doped (m,p<5e17 cm⁻³) x=0.65 AlGaAs 21.

The oxide aperture layers 24 consist of 20 nm thick x=0.98 AlGaAs 32,bounded on one side by 12 nm of x=0.97 AlGaAs 33, and x=0.65 AlGaAs 34on the other. This design yields a slightly tapered oxide “tip” in orderto reduce diffraction losses.

The process flow requires 6 photolithographic steps, represented by theindividual layers in sequence:

a. Intracavity contact etch

b. Mesa/trench etch

c. Isolation implant

d. Intracavity contact ohmic metallization

e. Topside ohmic metal

f. Thick interconnect and bondpad metal

FIG. 24 shows the overlay of each of the mask layers used to define eachfeature.

Referring to FIGS. 22 through 24, one possible fabrication procedureinvolves first depositing a ¼λ. SiN dielectric layer to passivate theoptical aperture and serve as contact protection during the wetoxidation step. Next, a Cl2-based dry etch using inductively coupledplasma reactive ion etching exposes the intra-cavity contact etch area35, followed by an HCL wet etch of the GaInP etch stop to expose theGaAs contact layer. Another ¼λ, SiN layer is then deposited to protectthe contact layer during the subsequent dry etch of the mesa trench area36 to expose the x=0.98 AlGaAs selective oxidation layers 32. A wetoxidation step turns the x=0.98 layers into aluminum oxide (Al₂O₃). Thisprocess is stopped before reaching the center of the mesa (˜10 μmnominal lateral oxidation distance) to form two oxide apertures 29′ and29″. In these apertures the aluminum oxide is insulating, but theunoxidized region is still conducting, and hence a guide for current iscreated. Following the oxidation, a high-energy (>380 keV) multi-stepproton implant through an 8 μm thick photoresist mask is used toelectrically isolate the devices from each other and intracavity anodefrom the cathodes. The photoresist mask creates an unimplanted region 37that protects the contact regions 7 and 9, as well as the deviceemitting aperture 13. Implanting after the mesa etch ensures completeisolation of the bottom DBR. The intracavity metal contacts 9 are thenpatterned for liftoff, followed by a CF₄ dry etch of the nitride anddeposition of the Ti/Pt/Au contact metal. After patterning anddeposition of the Ni/Ge/Au metallization of the topside ohmic contacts7, the top surface of the device is planarized with Cyclotene(BCB),followed by deposition of 1.6 μm thick Ti/Au metal forming theinterconnect metal and ground-signal-ground (GSG) coplanar waveguidecontacts 30 and 31. Finally, the wafer is thinned to ˜6 mils, metallizedwith Ni/Ge/Au to form the bottom terminal 8 metal contact and alloyedat >380 C to activate the ohmic contacts.

While the invention has been described with reference to an exemplaryembodiment(s), it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiment(s) disclosed, but that theinvention will include all embodiments falling within the scope of theappended claims.

What is claimed is:
 1. A laser system having separately electricallyoperable cavities for emitting modulated narrow linewidth light, saidsystem comprising: a compound semiconductor material substrate; at leasttwo first-mirror pairs of semiconductor material layers in a firstmirror structure on said substrate of a first conductivity type; a firstactive region on said first mirror structure with plural quantum wellstructures; at least twenty second-mirror pairs of semiconductormaterial layers in a second mirror structure on said first active regionof a second conductivity type; a second active region on said secondmirror structure with plural quantum well structures; at least twothird-mirror pairs of semiconductor material layers in a third mirrorstructure on said second active region of said first conductivity type,wherein reflectivities of the first mirror structure and the thirdmirror structure are substantially equal; an intermediate electricalinterconnection at said second mirror structure; and a pair ofelectrical interconnections separated by said substrate, said firstmirror structure, said first active region, said second mirrorstructure, said second active region, and said third mirror structure.2. The system of claim 1, further comprising a selected one of saidintermediate electrical interconnection and said pair of electricalinterconnections also being connected to a bonding pad supported on anelectrical insulator with said electrical insulator also supported bysaid substrate.
 3. The system of claim 1, wherein the laser system is avertical-cavity surface-emitting laser system, the system furthercomprising: a first electrical source operatively coupled to supplyforward-bias current to the first active region; and anelectrical-modulation source operatively coupled to vary theforward-bias current to the first active region.
 4. The system of claim1, wherein the first mirror structure includes a distributed Braggreflector that includes the at least two first minor pairs and the thirdminor structure includes a distributed Bragg reflector that includes theat least two third mirror pairs.
 5. The system of claim 1, wherein thefirst mirror structure includes a negative-conductivity-typesemiconductor distributed Bragg reflector, the second mirror structureincludes a positive-conductivity-type semiconductor, and the third minorstructure includes a negative-conductivity-type semiconductordistributed Bragg reflector.
 6. The system of claim 1, wherein themirror pairs of semiconductor material layers include alternating layersof two differing compositions of gallium, aluminum and arsenic.
 7. Thesystem of claim 1, wherein the plural quantum well structures of thefirst active region include alternating layers of two differingcompositions of gallium, aluminum and arsenic.
 8. The system of claim 1,further comprising: a plurality of current-confining oxidation layerseach having an aperture therethrough, including a first oxidation regionlocated on one side of the second mirror structure, and a secondoxidation region located on an opposite side of the second minorstructure.
 9. A laser system having separately electrically operablecavities for emitting modulated narrow linewidth light, said systemcomprising: a semiconductor material substrate; a first mirror structureon said substrate of a first conductivity type; a first active region onsaid first mirror structure; a second minor structure having at leasttwenty stacked pairs of semiconductor material layers located on saidfirst active region of a second conductivity type; a second activeregion on said second minor structure; a third mirror structure on saidsecond active region of said first conductivity type, whereinreflectivities of the first minor structure and the third mirrorstructure are substantially equal; an intermediate electricalinterconnection at said second minor structure; and a plurality of outerelectrical interconnections, a first one of which is electricallyconnected through the first minor structure to the first active region,a second one of which is electrically connected through the third minorstructure to the second active region.
 10. The system of claim 9,wherein the first minor structure is a distributed Bragg reflector thathas a plurality of layer pairs, the second minor structure is adistributed Bragg reflector, and the third minor structure is adistributed Bragg reflector that has a plurality of layer pairs, whereineach distributed Bragg reflector further includes a graded-compositionregion located between each layer of a plurality of the layer pairs ofone or more of the first, second, and third distributed Braggreflectors, wherein each graded-composition region changes incomposition to reduce electrical series resistance.
 11. The system ofclaim 9, wherein the first active region includes a plurality of quantumwells and the second active region includes a plurality of quantumwells.
 12. The system of claim 9, wherein the separately electricallyoperable cavities include a vertical-cavity surface-emitting laser and avertical-cavity electro-absorption structure coupled to one another bythe second minor structure.
 13. The system of claim 9, furthercomprising: a first electrical source operatively coupled to supplyforward-bias current-and a modulation signal to the first active region.14. The system of claim 9, wherein the first minor structure includes apositive-conductivity-type semiconductor distributed Bragg reflector,the second mirror structure includes a negative-conductivity-typesemiconductor, and the third minor structure includes apositive-conductivity-type semiconductor distributed Bragg reflector.15. The system of claim 9, wherein the minor pairs of semiconductormaterial layers include alternating layers of two differing compositionsof gallium, aluminum and arsenic.
 16. The system of claim 9, wherein thefirst active region includes a plurality of quantum well structureshaving alternating layers of two differing compositions of gallium,aluminum and arsenic.
 17. The system of claim 9, further comprising: aplurality of current-confining oxidation layers each having an aperturetherethrough, including a first oxidation region located on one side ofthe second mirror structure, and a second oxidation region located on anopposite side of the second minor structure.
 18. The system of claim 9,further comprising: a plurality of stacked mesa structures including afirst mesa structure having a perimeter sidewall surrounding the firstactive region and having a first diameter, and a second mesa structurehaving a perimeter sidewall surrounding the second active region andhaving a second diameter that is larger than the first diameter, whereinthe first mesa structure is formed on top of the second mesa structure.19. The system of claim 9, further comprising: a bonding pad formed on adielectric layer that is in turn formed on an implanted region ofGaAs-based semiconductor, wherein the implanted region is electricallyinsulating.
 20. A laser system having separately electrically operablecavities for emitting modulated narrow linewidth light, the laser systemcomprising: a semiconductor material substrate; a first means forreflecting light, wherein the first means for reflecting light islocated on the substrate, and wherein the first means for reflectinglight is of a first conductivity type; a first means for amplifyinglight, wherein the first means for amplifying light is located on thefirst means for reflecting light; a second means for reflecting light,wherein the second means for reflecting light includes at least twentystacked pairs of semiconductor material layers located on the firstmeans for amplifying, and wherein the second means for reflecting lightis of a second conductivity type that is different than the firstconductivity type; a second means for amplifying light located on thesecond means for reflecting light; a third means for reflecting lightlocated on the second active means for amplifying light, wherein thethird means for reflecting light is of the first conductivity type, andwherein reflectivities of the first means for reflecting light and thethird means for reflecting light are substantially equal; anintermediate means for conducting electricity connected to the secondmeans for reflecting light; a plurality of outer means for conductingelectricity, including a first outer means for conducting electricitythat is electrically connected through the first means for reflectinglight to the first means for amplifying light, and a second outer meansfor conducting electricity that is electrically connected through thethird means for reflecting light to the second means for amplifyinglight, wherein first means for reflecting light, the first means foramplifying light, and the second means for reflecting light form a firstVCSEL, and wherein second means for reflecting light, the second meansfor amplifying light, and the third means for reflecting light form asecond VCSEL; and means for applying a first constant-current value plusa modulation-current signal of forward-bias current to one of the groupconsisting of the first means for amplifying light and the second meansfor amplifying light.